document libs from sky_services (#3170)

* document libs from sky_services

* scan directory of sky_services for libraries

* simplify error message when sky_services directory not found

* address review comments

dev/dartdoc.dart

@@ -56,12 +56,33 @@ dependencies:
     args.add(name.substring(0, name.length - 5));
   }
 
+  _findSkyServicesLibraryNames().forEach((String libName) {
+    args.add('--include-external');
+    args.add(libName);
+  });
+
   process = await Process.start('pub', args, workingDirectory: 'dev/docs');
   _print(process.stdout);
   _print(process.stderr);
   exit(await process.exitCode);
 }
 
+List<String> _findSkyServicesLibraryNames() {
+  Directory skyServicesLocation = new Directory('bin/cache/pkg/sky_services/lib');
+  if (!skyServicesLocation.existsSync()) {
+    throw 'Did not find sky_services package location in '
+          '${skyServicesLocation.path}.';
+  }
+  return skyServicesLocation.listSync(followLinks: false, recursive: true)
+      .where((FileSystemEntity entity) {
+    return entity is File && entity.path.endsWith('.mojom.dart');
+  }).map((FileSystemEntity entity) {
+    String basename = _entityName(entity.path);
+    basename = basename.substring(0, basename.length-('.dart'.length));
+    return basename.replaceAll('.', '_');
+  });
+}
+
 List<String> _findPackageNames() {
   return _findPackages().map((Directory dir) => _entityName(dir.path)).toList();
 }
@@ -76,7 +97,6 @@ List<Directory> _findPackages() {
       return !nodoc;
     })
     .toList();
-
 }
 
 List<String> _libraryRefs() sync* {